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Post-9/11 USA is highly cautious of any perceived threat, Abta said

Holidaymakers have been warned to watch their words after two friends were refused entry to the US on security grounds after a tweet.

Before his trip, Leigh Van Bryan wrote that he was going to "destroy America".

He insisted he was referring to simply having a good time - but was sent home.

Trade association Abta told the BBC that the case highlighted that holidaymakers should never do anything to raise "concern or suspicion in any way".

The US Department for Homeland Security picked up Mr Bryan's messages ahead of his holiday in Los Angeles.

The 26-year-old bar manager wrote a message to a friend on the micro-blogging service, saying: "Free this week, for quick gossip/prep before I go and destroy America."

The Irish nationaltold the Sun newspaperthat he and his friend Emily Bunting were apprehended on arrival at Los Angeles International Airport before being sent home.

"The Homeland Security agents were treating me like some kind of terrorist," Mr Bryan said.

"I kept saying they had got the wrong meaning from my tweet."

No joke

Abta, which represents travel companies in the UK, said holidaymakers need to learn to be ultra-cautious when it comes to talking about forthcoming trips, particularly after 9/11.

Airport security staff do not have a sense of humour when it comes to potential riskAbta

"Posting statements in a public forum which could be construed as threatening - in this case saying they are going to "destroy" somewhere - will not be viewed sympathetically by US authorities," it told the BBC.

"In the past we have seen holidaymakers stopped at airport security for 'joking' that they have a bomb in their bag, thoroughly questioned and ending up missing their flights, demonstrating that airport security staff do not have a sense of humour when it comes to potential risk."

In another tweet, Mr Bryan made reference to comedy show Family Guy saying that he would be in LA in three weeks, annoying people "and diggin' Marilyn Monroe up".

Mr Bryan told the newspaper that he was questioned for five hours about his Twitter messages.

'Tweeter account'

After the interview, Homeland Security reported: "Mr Bryan confirmed that he had posted on his Tweeter website account that he was coming to the United States to dig up the grave of Marilyn Monroe.

"Also on his tweeter account Mr Bryan posted he was coming to destroy America."

The US Customs and Border Protection agency said in a statement that it tried to maintain a balance between "securing our borders while facilitating the high volume of legitimate trade and travel that crosses our borders every day".

It added: "We strive to achieve that balance and show the world that the United States is a welcoming nation."

Mr Bryan is not the only person to suffer from a misjudged tweet. In January 2010, Paul Chambers tweeted that he would blow snow-affected Robin Hood Airport in Doncaster "sky high!" if it was not reopened in time for him to see his girlfriend.

He was fined £385 plus £2,600 in costs - a sum which actor Stephen Fry offered to pay on Mr Chambers' behalf.